In many instances in the genetic engineering of plants in which a gene is inserted into the genome of a plant, it is desirable to be able to control expression of the gene. In other instances, it is desirable to be able to modify the expression of a gene which is native to the genome.
For example, in the creation of male sterility for the purpose of hybridizing crops, it is possible to select a gene which is normally required for pollen formation and inactivate that gene by insert a normally inactive promoter into the control sequences for the gene, so that the gene is "turned off" and the plant becomes male sterile, i.e., incapable of producing pollen. This system is disclosed in the copending application of Albertsen et al., Ser. No. 07/537,183, Filed Jun. 12, 1990. In order to reproduce plants of the modified genotype, it would be necessary to activate the promoter, desirably by some exogenous stimulus. Accordingly, a continuing need exists for promoters which are responsive to external stimuli such as non-phytotoxic chemicals.
In other instances it is desirable to place the expression of a gene under the control of a promoter which is responsive to unusual environmental conditions, such as drought and heat shock.